The term "tubular membrane module" is used to refer to a component which includes at least one tube comprising a semi-permeable membrane of tubular configuration supported on the inner surface of a porous pipe such as a glass fibre-reinforced fabric pipe. The tube is enclosed within an outer housing or "shell" having a permeate outlet. Fluid to be treated is passed through the tube under pressure. Permeate passes through the membrane and pipe into the interior of the housing and leaves through the permeate outlet. In some cases, a single tube is used, while in others the module may include a series of tubes arranged axially parallel to one another. A complete ultrafiltration apparatus, for example, will typically include at least one array of modules connected together so that fluid to be treated flows successively therethrough. The modules are usually replaceable individually.